1. Field of the Invention
This invention relates to a dropout detecting circuit for detecting the dropout in playback signals reproduced from an optical recording medium, and may be applied to, for example, an optical disk player employing so-called read only optical disks, write once disks or erasable optical disks.
2. Description of the Prior Art
In general, in a reproducing system for a video tape recorder or a laser disk player, for example, a signal dropout in the radio frequency (RF) playback or reproduced signals gives rise to excessive noise in the demodulated output and consequently to a deterioration of the quality of the image formed on the basis of the reproduced signals. For this reason, signal dropout detection in the RF playback or reproduced signals is carried out and steps are taken to compensate for the defective portion of the reproduced signals.
As a dropout detection circuit for sensing or detecting dropout of RF playback or reproduced signals, there is known a system operating on the principle of so-called zero-crossing. In such a system, the zero level (0 Lev) of the RF playback signals shown in FIG. 3 is detected, a pulse waveform having a corresponding period as shown in FIG. 4 is formed, and the dropout Do is detected from changes in the period. However, in this type of zero crossing, only larger dropouts causing complete elimination of RF playback signals can be detected.
Thus, in a more customary dropout detection circuit, portions Do of the RF signals exceeding a predetermined uppermost limit level +L.sub.2 or exceeding (in a negative direction) a predetermined lowermost limit level -L.sub.2, as shown in FIG. 5, are identified as representing signal dropouts. In other words, portions of the RF signals lying outside the range defined by +L.sub.2 and -L.sub.2 are identified as representing signal dropouts. Moreover, portions of the signal level of the RF signals not reaching an upper limit level +L.sub.1 or a lower limit -L.sub.1, as shown in FIG. 5, are also identified as representing dropouts.
In another conventional dropout detection circuit, portions Do of the playback RF signals exhibiting characteristic changes in the envelope thereof, as shown in FIG. 6, are identified as representing signal dropouts.
There has also been employed a circuit in which RF playback signals are demodulated into video signals, as shown in FIG. 7, and video signals Do appearing outside the video area defined by the broken lines are identified as representing signal dropouts.
In the RF playback signals from an optical recording medium, such as an optical disk, the frequency modulated playback video signals are superimposed on a predetermined offset DC voltage before being reproduced. When the RF playback signals are demodulated into video signals, the offset level is set to zero level (0 Lev). Thus the changes in the offset level caused by the material and the method of preparation of the disk or caused at the time of recording or playback contribute to the occurrence of dropouts that interfere with satisfactory signal reproduction.
However, in the conventional dropout detection circuits, signal dropout caused by fluctuations in the offset level can be detected only with difficulty. When an attempt is made to elevate the detection sensitivity in order to detect dropouts caused by fluctuations in the offset level, normal signals are likely to be erroneously identified as dropouts.